She operated along the east coast through the end of 1940 and into 1941. On her way to Pearl she set sail on 1 May for the Caribbean, emerged on the Pacific side of the Canal on 9 May and stopped off in San Diego through 21 May and arrived at Pearl Harbor on 31 May. She operated out of the Hawaiian Islands into the fall of 1941, as tensions rose in the Far East and the U.S. prepared for war in both oceans.

Thresher and sister-ship USS Tautog (SS-198)[?] departed the submarine base at Pearl on 31 October bound for a simulated war patrol north of Midway atoll. They both carried fully-armed torpedo warheads. Tautog returned first; and, on 7 December, Thresher neared the Hawaiian Islands to end her part of the cruise. Escorted by USS Litchfield (DD-336)[?] through Hawaiian waters lest she be mistaken for a Japanese submarine, Thresher received word at 0810 that Pearl Harbor was under attack by Japanese aircraft.

Litchfield promptly set off to join American light forces departing from the harbor, leaving Thresher alone to conduct her first real war patrol. However, the destroyer was ordered back to act as escort; radio contact between the two was established; and a rendezvous arranged.

At the pre-appointed time, Thresher poked up her periscope to have a look, and noticed a destroyer -- similar to Litchfield -- approaching, bows-on. Instead of a warm reception from friends, she got a hot reception from the destroyer's forward gunners, who opened fire on her as soon as her black conning tower broke the surface.

Thresher immediately went deep to avoid. She again tried to enter the harbor on the 8th, but was driven off by depth-bombs from a patrol plane, before USS Thornton (AVD-11)[?] finally arrived to provide safe-conduct for the boat at midday.

Departing Pearl Harbor on 30 December, Thresher headed for the Marshalls and Marianas. Reconnoitering Majuro, Arno, and Mili atolls from 9 January to 13, 1942, she shifted to waters off Japanese-held Guam in the early morning darkness of 4 February. A little before daybreak, a small freighter was sighted seven miles north of Agana Harbor and Thresher closed for the attack. She loosed a three-torpedo spread, holing the ship and sending it down by the bow and dead in the water. Thresher then fired another spread of torpedoes, but all missed. Upon returning to the scene one-half hour later the ship was gone and Thresher thought she had scored a kill, but postwar accounting did not substantiate it.

While enroute home to Pearl on 24 February, an overzealous Navy plane attacked Thresher but, did no damage and she safely returned to port on 26 February.

After refit, Thresher departed 23 March on her third war patrol which would take her near the Japanese home islands. There she was to gather weather data off Honshu for use by Halsey's task force centered around carriers USS Enterprise (CV-6) and USS Hornet (CV-8) then approaching the Japanese home islands. Embarked in Hornet were 16 Army B-25 medium bombers, under the command of Lieutenant Colonel James H. Doolittle[?], which would take off at sea and fly to Japan for strikes on Tokyo on 18 April. Enroute Thresher sighted a large Japanese freighter on the morning of 10 April. A three-fish spread was fired and all missed as the target escaped in the mist. When the target emerged from the murk Thresher was not in a position to launch another attack and proceeded on her way.

A second target was sighted later that day, and this time the hunting was better. One torpedo broke the back of a 3,000-ton freighter off Yokohama harbor, sending it to the bottom in less than 3 minutes. The subsequent depth charge attack caused Thresher to lose depth control and she began plunging towards unexplored depths before her bow was trimmed, and control regained.

On the 13th, running on the surface to recharge her batteries, Thresher took a high wave over her Conning Tower. Water cascaded down the open hatch and rushed into the boat, grounding many electrical circuits. For a short time, there was a decided danger that chlorine gas would be released endangering the crew, but quick thinking and damage control prevented that from developing. Eventually, all shorts were repaired; and the boat pumped out.

On the 14th, Thresher departed her assigned patrol area and turned her attention to gathering weather data. She conducted periscope patrols in the advance screen of Halsey's task force, searching for any enemy craft that could warn the Japanese homeland. She was detached from this duty on the 16th and, after evading two Japanese patrol planes, returned to Pearl on the 29th.

On 26 June, Thresher commenced her fourth war patrol heading for waters between the Palaus and the Marshalls. On 6 July one torpedo struck home during an attack on a tanker off Enijun Pass. The two surface escorts were soon joined by aircraft and Thresher was able to escape after a three-hour depth bomb counterattack to resume her search for other targets.

Midway between Kwajelein and Wotje atolls, Thresher fired two topedoes at a 4,836-ton torpedo boat tender which caused tremendous explosions as the tender sank beneath the waves. Thresher withdrew from expected countermeasures. Within an hour, two depth charges shook the boat, and ten minutes later, a banging and clanking alerted her to the fact that the Japanese were apparently bringing a large grapnel into play in an attempt to capture the boat.

Thresher was hooked and fought for her life. After applying full right rudder, she made a ten-minute, high-speed run which shook her free from the giant hook. Then, as a depth charge exploded near her Conning Tower, the boat went into deeper water. What a bending and twisting turn, Thresher left the enemy behind, with some 30-odd depth charges exploding in her wake. Shaken but not seriously damaged, Thresher made minor repairs as she headed for Truk to reconnoiter the passes leading into this enemy naval bastion.

Missing a freighter with torpedoes on the night of 20 July, Thresher surfaced in a rain squall before daybreak on the 21st. The boat's sonar picked up the sound of screws, close and closing. Soon an enemy patrol craft came into view, approaching on a collision course. Surprisingly, the Japanese chose not to ram, but instead put over his helm hard right, and came to a parallel course some 50 yards away. Thresher "pulled the plug" to dive deep, while the enemy's guns fired close but ineffective salvoes into the water ahead of the disappearing boat.

After escaping by silent-running to the Palaus, Thresher tangled with an enemy Q-ship off Ambon in the former Netherlands East Indies. The two torpedoes that she fired at the enemy failed to detonate, and the Q-ship subjected Thresher to an eight-charge barrage before giving up the attack. Since she had been reassigned to the Southwest Pacific Submarine Force, Thresher sailed away from this encounter enroute to Australian waters and terminated her fourth war patrol at Fremantle on 15 August.

After refit, Thresher loaded mines and departed Fremantle on 15 September, bound for the Gulf of Siam. She fired torpedoes at two freighters north of Lombok Strait on 19 September but was unable to determine the results of her attacks. On the night of the 25th, luck again failed to smile on her as a single torpedo streaked beneath a large, high-speed target in the Sulu Sea.

Thresher later surfaced at 2300 and proceeded on a course which took her north to Pearl Bank. There, in the northernmost reaches of the Gulf of Siam, she made one of the first mine plants by a submarine in the Pacific war. These strategic "plants" by Thresher and her sisters, in subsequent patrols, covered Japanese shipping lanes in areas of the Southwest Pacific Command previously unpatrolled by submarines. Later, these minefields filled the gap between patrol zones along the coastal waters of Malaya, Siam, and Indochina, when many boats were diverted to participate in the Solomons campaign.

While reconnoitering off Balikpapan, Borneo, and the Celebes coast, Thresher sighted a tanker aground on a reef off Kapoposang Island in the Java Sea. She soon surfaced for a deck-gun attack and left the enemy ship with decks awash. The boat then returned to her base at Fremantle on 12 November for refit.

Underway from Fremantle on 16 December, she arrived off Soerabaya, Java, on Christmas Day. She intercepted a convoy of freighters, escorted by two destroyers, several subchasers, and two aircraft. Slipping past the escorts, Thresher sent five torpedoes towards the leading three ships. Two successive explosions followed. Rising to periscope depth, the boat observed the second ship in the column down by the bow, with her stern up in the air and her screws, still revolving, out of the water. A second ship lay dead in the water, enveloped in smoke. Escaping unscathed from this tangle with a coastal convoy, Thresher sighted an enemy aircraft carrier the next night, but was picked up by escorts and held at bay for more than an hour while the tempting task force faded into the night.

On the night of 29 December, Thresher made contact with a 3,000-ton freighter. Just before midnight, she fired a spread of torpedoes at the cargoman; but all missed or ran too deep. Undaunted, she waited for the moonrise and then surfaced to utilize her deck gun. Outmaneuvering the enemy, who tried to ram her, Thresher scored eight hits in succession with her 5-inch main battery, then finishing it off with a single torpedo.

After arriving back in Fremantle on 10 January1943, the boat got underway 15 days later for her seventh war patrol. At 1100 on 14 February, Thresher made contact with a Japanese I-65-class submarine to the east of Thwartway Island. She launched two torpedoes; one was a dud, and the other exploded on the ocean bottom. Turning north and firing deck guns, Thresher's adversary soon disappeared from sight over the horizon.

Proceeding to the Flores Sea, she intercepted a three-ship convoy escorted by two anti-submarine vessels on 21 February. Loosing a two-torpedo salvo, Thresher scored a hit with one on the stern of a transport. Thresher then evaded 13 depth charges before returning to periscope depth a little more than an hour later. She observed her target lying dead in the water while barges carried troops from the first transport to an undamaged sister. As escorts searched the waters nearby, Thresher closed and torpedoed the second transport as it was lying-to during the transfer of survivors from the first. Two loud explosions reverberated in the background as the boat dived to avoid possible countermeasures.

The following day, Thresher returned to celebrate Washington's Birthday by finishing off the first transport which jack-knifed into a "V" shape and sank within three minutes.

Thresher prowled for more game and came upon a tanker and a freighter on 2 March. A single torpedo hit on the 5,232-ton tanker and it sank. The freighter, sighting torpedo wakes, took evasive action to avoid being hit. Then, a nearby escort arrived on the scene and kept Thresher at bay while the target escaped. The boat subsequently concluded this patrol arriving at Fremantle on 10 March.

Her eighth war patrol from 4 April to 23 May was uneventful, but her ninth saw the boat score another "kill." Off Balikpapan, Borneo, she sighted a three-ship convoy, escorted by a sole destroyer on the night of 30 June. After an unrewarding try with a trio of torpedoes, Thresher dodged the escort's depth charging attack and returned with the scent of a kill. Tracking with radar, Thresher set a tanker ablaze from stem to stern and scored hits on a 5,274-ton passenger-freighter, which sent the hapless merchantman to the bottom of Makassar Strait.

Heading for Tambu Bay on the morning of 5 July, Thresher tracked a tanker. Chasing her quarry along the Celebes coast, the submarine lurked nearby until the escort left. Thresher then closed, loosed three torpedoes, and scored one hit on the bow of the enemy vessel. However, this blow failed to stop the tanker, which fired her guns to keep Thresher at bay as she escaped at high speed.

Four days later, Thresher arrived off Catmou Point, Negros Island. Under cover of darkness, the boat surfaced and delivered 500 pounds of stores and 40,000 rounds of ammunition to Filipino commandoes. Receiving intelligence documents in return, Thresher got underway for a resumption of her patrol shortly before midnight on 9 July. She soon departed the Philippines and sailed via Midway and Pearl Harbor to the west coast for a major overhaul at the Mare Island Naval Shipyard, Vallejo, California.

Newly refitted, Thresher departed the west coast on 8 October and arrived at Pearl one week later and commenced her tenth war patrol as she departed the Hawaiian Islands on 1 November, bound for the waters north of the Carolines. Prowling north of Truk, Thresher commenced tracking a five-ship convoy on the morning of 12 November and slipped past two escorts shortly before midnight.

She fired three "fish" into a 4,862-ton transport. The next attack failed, as a second trio of torpedoes missed their mark. Escorting antisubmarine craft hunted in vain for the American attacker, dropping 20 depth charges in an harassing barrage.

Thresher's 11th war patrol took her to the South China Sea below Formosa. While cruising on the surface on 10 January1944, Thresher sighted a pair of masts, low on the horizon, and quickly dove to avoid possible detection. Coming to periscope depth soon thereafter, she approached cautiously, keeping in mind that the ship may have been the advance screen of a convoy. The contact proved to be a 150-ton trawler. Thresher battle-surfaced, and her gun crews tumbled out on deck to man the guns. Opening the action from 6,000 yards, she expended 45 five-inch shells; 1,000 rounds of 50-caliber machinegun fire; and 770 20-mm shells. Finally, the trawler went down. The boat's war patrol report noted the tonic effect on the crew. Thresher's C.O. commented in this patrol report: "Not much damage was done to the Imperial war effort, but the action had a good psychological effect on the crew."

Thresher next set course for the Luzon Strait, between Batan Island[?] and Luzon, in the Philippines. At 1143 on the 15th, Thresher came to the surface and spotted a Japanese aircraft carrier with an escorting destroyer soon thereafter. The boat "pulled the plug" but soon came up to periscope depth to observe the enemy destroyer rapidly approaching. With insufficient time to maneuver for a "down the throat" shot, Thresher went deep and rigged for silent running. The destroyer churned overhead and dropped four depth charges -- none of which fell very close to the boat. After remaining above for two hours, the escort finally turned away, leaving Thresher unscathed.

Again coming to periscope depth at 1700, Thresher soon sighted a four-ship convoy at 12,000 yards with a sole sub-chaser as escort. Surfacing at 1911, Thresher began the chase, tracking the convoy by radar. The three leading targets steamed in column formation, roughly 500 to 800 yards apart, with the fourth some 6,000 yards astern. The escort was between the third and fourth merchantmen. Approaching from the westward, to take advantage of the moonrise, Thresher stalked her prey, whose behavior had been, to a point, predictable.

Previously zigzagging every 10 minutes, the convoy changed course at 2155 -- giving Thresher an excellent setup for her stern tubes. At 2207, the boat let fly with four torpedoes from 1,800 yards at the lead ship, a 6,960-ton freighter. Thresher observed two hits; and the vessel, with her bow in the air, was observed in a sinking condition.

Thresher next fired three bow tubes at the second target, a 4,092-ton freighter. Three torpedoes struck the ship -- evidently a tanker -- and literally blew her to pieces. The cargo of oil burst into flames and illuminated the night as brightly as day.

The third ship commenced fire with deck guns on Thresher, passing down the port side at 800 yards. With the boat now illuminated by the burning oil, and with her after tubes spent, Thresher's historian noted later that "our usefulness for the moment was over." Accordingly, Thresher dove as bullets from the approaching escort splashed nearby.

Thresher counted some 20 explosions from depth charges before the patrol craft left an hour later. Upon surfacing, Thresher was again alone and set off to patrol along the Singapore-to-Japan trade route.

On 26 January, Thresher made radar contact with a small convoy. Closing her sighting, Thresher soon spotted two ships steaming along beneath the overcast night skies. At 0011, Thresher fired three bow torpedoes at a 1,266-ton freighter, before the boat bent on full-speed ahead and hard right rudder to clear the area. Her "fish" scored a bullseye, and the quarry disappeared within a minute.

A secord spread of torpedoes, fired 35 seconds after the first, plowed into a second 2,205-ton freighter, which sank soon thereafter. A third target made off to the south at high speed, "spraying the ocean with 5-inch ammunition." Resuming the approach at 0020, Thresher doggedly tailed the Japanese freighter for four hours before reaching a favorable attack position. Firing her last torpedoes at 0446, Thresher began to build up speed and had just commenced a turn when one torpedo struck the enemy ship, causing a tremendous explosion.

The blast slowed the freighter, but its tremendous concussion stopped Thresher dead in the water. All four main engine overspeed trips were actuated; cork insulation flew; lights broke; clocks stopped; and water poured down the antenna trunk. By the time Thresher regained battle readiness, the enemy was too far away to encourage further pursuit.

Well within the range of shore-based aircraft, Thresher quit the chase. Escorts, alerted to the fact that an American submarine was prowling in the vicinity, arrived on the scene and conducted a three-hour long, but futile, depth-charging. On the 28th and 29th, Thresher patrolled the Formosa-to-Palau shipping route, in the area of the Luzon Strait, before returning via Midway to Pearl where she arrived on 18 February. There, Lcdr MacMillan was awarded the Navy Cross for his aggressive action during the patrol.

Thresher went to sea on 18 March, departing Pearl for the central Carolines. She remained on air-sea rescue station during American carrier strikes on Truk, bombarded Oroluk Atoll on 11 April, and photographed islands in that group. The boat played "hide and seek" with numberous enemy aircraft and witnessed several American bombing raids of Truk. She sighted only two enemy ships and was unable to attack either, before she returned to Pearl Harbor on 8 May.

On 14 June, Thresher headed out for her 13th war patrol. She joined a wolf pack -- consisting of USS Apogon (SS-308)[?], USS Guardfish (SS-217)[?], and USS Piranha (SS-389)[?] -- on the 25th. Nicknamed the "Mickey Finns" and under the overall command of Captain William V. O'Regan, the group picked up "ditching signals" from a downed aircraft that afternoon and changed course to investigate. Arriving in the vicinity on the 27th, they found only a drop tank and no trace of plane or pilot.

Over the succeeding days, the boats observed several planes but contacted only a few fishing vessels and small patrol craft. This drought of targets continued until 11 July, when Thresher made radar contact with a group of six ships steaming on the Formosa-Luzon route. As she changed course to intercept, she dispatched contact reports to the other boats. Guardfish and Apogon picked up the contact, but Piranha could not. Thresher deployed to a position 15,000 yards astern of the convoy, to trail the enemy group and be ready to pick off stragglers. Guardfish took the enemy's port flank. Apogon maneuvered to the convoy's starboard quarter.

A Japanese escort latched on to Thresher, however, and trailed her, depriving her of a chance to attack the convoy. Meanwhile, Piranha managed to sink a 6,504-ton passenger-cargo ship, while Apogon was rammed and forced to return to her base for repairs.

Rendezvousing on the 13th, the remaining boats resumed their hunting. At 1600 on the 16th, Thresher sighted smoke on the horizon. She surfaced and dispatched a contact report. After a cat-and-mouse period of some two hours; she noted that the convoy consisted of six ships: a large tanker, three freighters, and two escorts.

Thresher closed her prey beneath a clear and dark night sky. At 2329, with the range to the near escort at 2,000 yards, she opened fire. Three torpedoes sped from the forward tubes at the lead escort; three at the first freighter. Lcdr MacMillan then turned Thresher 150 degrees to port and launched four torpedoes at the second freighter. Four explosions were sighted and as she departed at high speed, six were heard soon thereafter.

Commencing her reload at midnight, Thresher returned to the area and continued the attack on the convoy which consisted now of only three ships: a freighter, the oiler, and an escort. At 0118, she fired two bow tubes at the escort and three at the leading freighter. She then swung 165 degrees to starboard to fire stern shots at the oiler. Soon thereafter, she heard at least six explosions. The escort promptly began a depth charge barrage. Returning to periscope depth, she found that the convoy had remained stubbornly afloat. Commencing another reload at 0122, she stalked her crippled prey.

While tube number six was being reloaded, Thresher fired two bow tubes at the freighter; two at the oiler; and one at the escort. Then, Thresher swung about and fired one stern tube at the latter. Two torpedoes exploded at 0246, and the cargo ship sank immediately. One minute later, two "fish" struck the oiler. A tremendous explosion lighted the entire sky, and the ship sank within 15 seconds.

While it could not be ascertained whether or not the last escort went down, the effect of two torpedo hits made it likely that she had been heavily damaged. All torpedoes expended, Thresher headed for Midway. The boat claimed to have destroyed the entire convoy, but a port-war assessment only credited her with two cargo vessels: one of 4,916-tons and the other of 2,838-tons. Thresher did, however, receive the Navy Unit Commendation for the patrol.

Upon completion of voyage repairs, Thresher stood out of Midway on 23 August, bound for the Yellow and East China Seas on her 14th war patrol. Six days later, while cruising on the surface, Thresher was battered by heavy seas which caused the boat to roll some 53 degrees from the vertical and produced waves up to 50 feet high.

Rounding the southern tip of Kyushu, Thresher sighted several small craft before making contact with a minelayer and two subchasers on the 10th. Clearing the vicinity at high speed, Thresher headed for new patrol grounds.

Thresher was twice frustrated on the 13th, when a large oiler passed far out of reach and a freighter -- attacked with four torpedoes -- remained stubbornly afloat. An escorting aircraft harried the boat and prevented any further attacks.

At 1531 on 18 September, Thresher sighted the masts, funnel and bridge of a ship on the horizon. After determining the enemy's base course and zigzag plan, Thresher surfaced and locked on the freighter with radar at 1923. Another pip, an escort vessel, soon appeared on the radar screen.

By 2100, Thresher had manuevered into position off the enemy's port bow and waited for the Japanese ships to make a zig which would place her at a desirable point for the attack. Thresher closed in for the kill and loosed four torpedoes as the group turned to the right. The quarry, however, elected to move contrary to the hunter's expectations; and the first spread ran wide of its targets. Thresher, still undetected, quickly came about and fired four stern "fish" from 1,200 yards.

The second spread ran true and struck a 6,854-ton freighter. The explosions broke the cargoman's back, and she quickly slipped from sight. Thresher retired at high speed when she detected the presence of three additional ships -- including light cruiser Yubari[?] -- closing rapidly.

Reloading, Thresher turned upon her pursuers, loosing a spread of torpedoes which barely missed. Subsequently evading her hunters and shifting to waters off Manchuria, the boat sighted only fishing craft until the 26th, when a large cargo vessel hove into sight at 0944. Thresher surfaced at 1315 and headed for the nearest point on the enemy's zigzag course. An hour later, the boat spotted a floatplane on patrol, and hurriedly "pulled the plug" to submerge. As she went deep, one depth charge exploded nearby.

Staying under until 1600, Thresher came to the surface and regained her target at 1815. Tracking her quarry until sunset it was postulated the enemy vessel was bound for Daisei Gunto and an intercept course was plotted accordingly.

Attacking from the bright moon side, Thresher fired two bow tubes -- aiming one torpedo at the hull near the mainmast and one at the fore. Both struck home, and the 1,468-ton freighter broke up and sank within a minute.

The following day -- 26 September -- Thresher came upon a 5,000-ton oiler and cut loose with four stern tubes from a range of 4,000 yards. Those on the bridge saw the target disappear within a minute. Empty of torpedoes, Thresher headed for Midway. En route, on 3 October, she sighted, tracked, and approached a small trawler. After sunset, Thresher surfaced and manned her deck guns. After firing 27 rounds of 5-inch ammunition, the boat soon received an answer in the form of shells which fell around the boat and forced her to back off. Too dark to see the target, Thresher resumed her passage to Midway.

After fueling at Midway on 8 October, Thresher sailed for the Hawaiian Islands and arrived at Pearl on the 12th. Following a lengthy refit, Thresher got underway on 31 January1945 and proceeded to the Marianas in company with USS Tilefish (SS-307)[?], USS Shad (SS-235)[?], and USS Peto (SS-265)[?]. Remaining at Saipan overnight, 12 February and 13, the "wolf pack" pushed on toward its assigned patrol areas north of Luzon in the Philippines. However, Thresher's 15th war patrol proved to be largely unproductive. Only two of her contacts developed into attacks. One failed due to the target's shallow draft; and, in the second, the quarry shook off the hunter with evasive manuevers. Thresher did, however, conduct air-sea guard patrols; and conducted a shore bombardment of Basco Harbor, Batan Island, on 28 March. The latter part of this patrol was conducted in company with Piranha and USS Puffer (SS-268)[?].

Clearing her patrol station, Thresher nested alongside USS Fulton (AS-11)[?] for voyage repairs before pushing on for Oahu on 4 April. Arriving at Pearl 20 days later, Thresher thus ended her active combat service upon conclusion of her 15th war patrol. Undergoing a routine refit and voyage repairs, Thresher subsequently rendered target training services out of Pearl and Eniwetok. She was operating out of the latter base on 15 August1945 when the war in the Pacific ended.